The present invention relates to a circuit for protection of the output stage of an intelligent power actuator from overloads and short circuits. The need for protecting these devices which can be switches, amplifiers and voltage or current regulators appears obvious when the power in play becomes high because an overload or an accidental short circuit could irreparably damage both the load and the device.
In many applications and in particular in current regulators it is often required that the limitation current be programmable from outside the integrated circuit.
The output stage of the device which is the subject of the present invention is provided by means of a power transistor and the technique used for limiting the current is that of driving the above mentioned transistor with a negative feedback network which tends to inhibit the transistor when the current running therein exceeds a certain predefined threshold. The simplest way to detect the current is that of measuring the voltage drop on a resistorxe2x80x94termed xe2x80x98sensexe2x80x99 resistorxe2x80x94placed in series with the power transistor.
However, in the case of xe2x80x98low dropxe2x80x99 applications, i.e. in which the voltage drop on the power transistor must be very low, this technique has the obvious disadvantage of increasing the voltage drop and requiring, as a sense resistor, a resistor with power structure.
In these cases it is possible to use a circuit solution like the one illustrated in FIG. 1 in which is shown a current limitation circuit in a final stage of a power actuator. In this circuit the limitation circuit is proportional to a reference current obtained through a variable resistor Rext which is normally outside the integrated device. The output current Iout is divided and only part of it is measured through a sense resistor RS connected in series with a sense transistor PS with the obvious advantage that the output resistance is not changed.
The transistor PS, termed xe2x80x98power sensexe2x80x99, must be well coupled with the power transistor PW and is sized with an area n times smaller than the total area occupied by both the transistors. In this manner when both the transistors PW and PS work in the saturation region the current running in the transistor PS is Is=Iout/n.
The voltage drop at the ends of the resistor RS is compared by a comparator 3 with a reference voltage obtained by running a current IR of known value in a resistor RR. In particular the comparator 3 is an operational amplifier whose output is connected to the control terminals of the transistors PS and PW.
The reference current IR is generated inside the device by the circuit block 4 which generates a current proportional to the external resistor Rext.
In this circuit solution the output current Iout is limited when it reaches the value IL=K*IR where the multiplying factor K depends on the relationship of the resistors RR and RS and on the area relationship n of the transistors PS and PW in accordance with the following formula.   K  =      n    ·                  R        R                    R        S            
The reference current IR generated by the circuit block 4 is inversely proportional to the variable resistor Rext       I    R    =            V      R              R      ext      
Hence, in the case of a short circuit or an overload the limitation current IL will be:       I    L    =      K    ·                  V        R                    R        ext            
As may be readily seen from this last equation, the circuit block 4 is to be implemented in such a manner as to prevent the limitation current from becoming too high if, intentionally or by error, the value of the resistor Rext drops too much or is zeroed.
Two circuit solutions for generating a reference current IR inversely proportional to the value of a resistor are proposed by B. Gilbert in the article xe2x80x9cA Versatile Monolithic Voltage-to-Frequency Converterxe2x80x9d published in IEEE Journal of Solid-State Circuits in December 1976 and J. F. Kukielka and Solid-State Circuits in December 1976 and J. F. Kukielka and R. G. Meyer in the article xe2x80x9cA High-Frequency Temperature-Stable Monolithic VCOxe2x80x9d published in IEEE Journal of Solid-State Circuits in December 1981.
In the above mentioned circuits, termed by the authors xe2x80x9cVoltage to current converterxe2x80x9d, the reference current is not limited and becomes high as the value of the resistor Rext decreases.
To obviate this problem in the prior art the circuit block 4 is commonly implemented starting from the operating principle of the circuits proposed by Gilbert, Kukielka and Meyer with the addition of other circuitry having the function of limiting the reference current.
A practical embodiment of the circuit block 4 in accordance with the prior art is shown in FIG. 2. In the diagram of FIG. 2 there can be distinguished the following circuit parts.
a MOS transistor N1,
a current generator IQ,
a regulation circuit 10 consisting of an operational amplifier 8 and a MOS transistor N2, and
a limitation current circuit 11 consisting of an operational amplifier 9, a MOS transistor N3, a reference voltage V1 and a resistor R1.
Therein can be distinguished two negative feedback loops, the one introduced by the regulation circuit 10 and the one introduced by the limitation current 11. The first feedback loop has the function of regulating the voltage on the circuit node A in such a manner that the current IR is proportional to the resistor Rext.
The second feedback loop has the function of limiting the current IR when the value of the resistor Rext falls below a certain value. In particular the current IR never exceeds the maximum value given by the following formula.       I    RMAX    =            V      1              R      1      
The present result applied to the equation of the limitation current IL gives the maximum value which the limitation current can assume.
xe2x80x83ILMAX=Kxc2x7IRMAX 
The above mentioned solution however presents in principle the characteristic of having two negative feedback loops and this characteristic involves a considerable circuit complexity and nearly always the need for having compensation capacitors inserted in the feedback loop with the obvious disadvantage of waste of surface area on the integrated circuit.
The technical problem underlying the present invention is to generate a reference current for a circuit limiting the maximum current delivered by a power transistor and inversely proportional to the value of a resistor and at the same time self-limited while utilizing a single feedback loop.
This technical problem is solved by a circuit for limitation of the maximum current delivered by a power transistor of the type indicated and defined in this specification.
The technical problem is also solved by a power actuator protected at output from overloads and short circuits of the type indicated and defined in this specification.